1. Field of the Invention
This invention relates generally to fiber optic components, and in particular to a polarization-independent in-line isolator utilizing a beam aperture and/or an offset beam aperture method.
2. Description of Related Art
Optical isolators are one of the most important passive components in telecommunications, optical sensing and related technologies. The function of an optical isolator is to let a light beam pass it in one direction, that is, the forward direction only (like a one-way traffic). The isolator dramatically suppresses the light propagating in the backward direction.
The current polarization-independent fiber optic (in-line) isolator is based on the Faraday effect, which is one of the most commonly used nonreciprocal effects. The key part (sometimes called the core) of a stage of the art fiber optic isolator consists of a Faraday single crystal (for example, Garnet), a hollow magnet and two birefringent crystals (for example, LiNbO.sub.3). The Faraday crystal is placed inside the hollow magnet and between these two birefringent crystals. Collimated beams are usually required as incident beams for the core from the forward and backward directions. Hence, a fiber collimator comprising basically a fiber and a Grin (graded index) lens is used at the input and output ends of the isolator.
The forward-going beam from the input fiber, upon impinging on the first birefringent crystal, becomes two spatially separated beams (the ordinary and extraordinary beams with orthogonal polarization vectors, that is, the O-beam and the E-beam) due to the birefringence. After passing the Faraday crystal and the second birefringent crystal, these two beams are paralleled to each other and therefore fully collected by the second collimator. However, the O-beam and the E-beam of the backward-going beam emerging from the first birefringent crystal, become divergent due to the nonreciprocal Faraday effect. They cannot be collected by the first collimator. Thus, the function of an isolator (to suppress the backward-going beam) is realized. The structure of such an isolator is quite complicated, and accurate optical alignments between these parts are required. The high cost of the core and labors leads to a relatively expensive isolator.
There is therefore an urgent need for another type of fiber optic isolator that significantly reduces the number of components, resulting in a simpler structure, a faster assembly time, lower cost, higher reliability and improved performance.